This invention relates to ducting.
The invention is more particularly concerned with ducting having sound absorbing properties.
Ducting with sound absorbing properties is often used in applications where it extends from a high noise environment, so as to prevent transmission of this noise. To achieve the greatest sound absorbing properties, the ducting should have a high attenuation of orifice noise, that is, noise transmitted along the length of the ducting through its bore, and a high attenuation of radiated noise, that is, noise transmitted through the wall of the ducting. It is difficult to provide low cost, lightweight ducting having a high attenuation of both orifice and radiated noise. In some cases, the ducting may require greater orifice attenuation and lower radiated attenuation, whereas in other cases the ducting may require greater radiated attenuation and lower orifice attenuation. The need to make specialised ducting for these different applications adds to the cost of manufacture. Ducting can be provided with an inner layer of a sound absorbing foam, such as described in GB2218772. In this previous ducting, overlapping parts of a helical foam strip are crimped into a helical metal channel and this is covered by an outer impervious sleeve shrunk about the channel.
It is an object of the present invention to provide improved ducting.
According to the present invention there is provided ducting comprising an inner support member of helical shape, a first layer wound helically about the outside of the support member and bonded thereto, and a second layer wound helically about the outside of the first layer, the first and second layers being both of a permeable, open cell thermoplastic rubber material.
The first layer preferably has a substantially higher permeability than the second layer. The first layer may have a multiplicity of holes between 2 mm and 3 mm in width, the second layer having fewer, smaller holes less than about 1 mm in width. The first and second layers are preferably of substantially the same chemical composition. The first and second layers are preferably produced by extrusion and the extrusion output of the first layer is preferably lower than that of the second layer. The first layer may be wound onto the inner support member while warm so that it bonds with the support member. The first layer may be provided by a strip wrapped on the inner support member with the same pitch and overlapping approximately half the width of adjacent turns. The inner support member may be a plastic-coated wire. The ducting may include a plurality of yarns of non-elastic material extending axially along the ducting between the two layers. The ducting may include a yarn of non-elastic material extending helically around the ducting between the two layers, which is preferably wound on top of the axially-extending yarns.